1. Field of the Invention
This invention is in the field of ceramic compositions having high dielectric constants and including strontium titanate as the major ingredient, the crystalline lattice of the strontium titanate being substituted by oxides of niobium, tantalum, germanium, or zinc and having insulated grain boundaries through the diffusion therein of either bismuth oxide or a combination of bismuth oxide, lead oxide, and boric oxide.
2. Description of the Prior Art
It has heretofore been known that it is possible to produce a ceramic dielectric with a large effect dielectric constant by insulating the grain boundaries of a conventional ceramic semiconductor. This type of structure is generally referred to as a "boundary layer type capacitor." A ceramic semiconductor of the barium titanate series, for example, an effective dielectric constant of 50,000 to 70,000 can be obtained, with an insulation property such that its withstand voltage is 800 volts per millimeter and its insulation resistance is about 2 .times. 10.sup.11 ohm/cm. The difficulty with this type of capacitor, however, is that it evidences a capacity change of about .+-. 40.degree. from its value of 20.degree. by a change in temperature to the range of 30.degree. to 85.degree.C. Furthermore, its loss factor is comparatively large, being at about 5 to 10%, so that its usefulness as a capacitor is restricted.
It has also been known that strontium titanate can be used as a principal component of a capacitor, when combined with manganese oxide and silica, and the resultant mixture is sintered in an argon atmosphere to obtain a ceramic semiconductor. This structure, with or without an additional manganese oxide coating, can be subjected to a secondary sintering in an oxidizing atmosphere to cause the grain boundaries to become insulated. The product thus obtained is as low as 15% in electrostatic capacity change in the range of 30.degree. to 85.degree.C, and its loss factor becomes as low as 2 to 5% so that these properties are better than the corresponding properties of a barium titanate type. However, the effective dielectric constant at a withstand voltage of 1,800 to 1,000 volts per millimeter is only about 20,000 to 35,000, which is very low.